This list contains awesome OS related stuff. It contains open source operating sytems and hobby operating systems as one can study their code and learn from them.
- BoneOS- OS for everyone built by everyone
- Clive - A unikernel OS inspired by Plan9 and Nix developed at Universidad Rey Juan Carlos of Madrid
- Genode - A FOSS operating system framework consisting of a microkernel abstraction layer and a collection of userspace components
- HelenOS - multikernel multiserver OS
- KnightOS - for z80 calculators
- L4re - Operating system and hypervisor for security/safety-critical and virtualization-enabled applications.
- Minoca OS - General purpose OS, written in C
- Redox - written in Rust
- Sculpt OS - Genode based OS
- Thor - 64bit operating system mostly written in C++
- Interim - Minimalist OS with concepts from Lisp machines and Plan9
- FreeRTOS, GitHub - IoT operating system for microcontrollers, by Amazon.
- Cyjon - pure x64 assembly language operating system
- MyXomycota - Monolithic kernel in C
- Serenity - Graphical Unix-like operating system for x86 computers
- Snowdrop OS - a homebrew operating system from scratch, in assembly language
- Sortix - Hobby OS in C and C++ by Jonas 'Sortie' Termansen
- ToaruOS - Hobby operating system from scratch
- Týndur - Hobby operating system build by the Lowlevel community. Written in C and Pascal
- TempleOS - Religious OS :-)
- Ultron OS - x86 Operating System written in C++, High School Project
- Xv6 - A teaching operating system developed in the summer of 2006 for MIT's operating systems course
- How to Make a Computer Operating System - in C++
- Intermezzos - A follow along book to build IntermezzosOS in Rust
- The little book about OS development by Erik Helin and Adam Renberg
- Think OS - A Brief Introduction to Operating Systems by Allen B. Downey
- Operating System Development Series - OS from the ground up in C
- Operating Systems: Three Easy Pieces - Easy to read book covering virtualization, concurrency and persistence
- xv6: a simple, Unix-like teaching operating system - Book for Xv6
- How to write a Linux kernel patch and submit it by Greg Kroah-Hartman
- Presentation on how the Linux kernel is developed by Greag Kroah-Hartman
- os-tutorial - How to create an OS from scratch by Carlos Fenollosa
- Writing an OS in Rust by Philipp Oppermann
- Kernels 101 – Let’s write a Kernel by Arjun Sreedharan
- Kernels 201 - Let’s write a Kernel with keyboard and screen support by Arjun Sreedharan
- os-dev - Wiki with everything you need to know
- Lowlevel - A wiki about creating an OS from scratch. In German
- POSIX - Standards for maintaining compatibility between operating systems
- The benefits and costs of writing a POSIX kernel in a high-level language by Cody Cutler, M. Frans Kaashoek, and Robert T. Morris, MIT CSAIL